The present invention relates generally to nuclear reactors, and, more specifically, to electrochemical corrosion potential (ECP) sensors therein.
In a boiling water nuclear reactor, water and steam are channeled through various conduits formed of stainless steel. Normal water chemistry conditions include high oxidizing species, such as oxygen and hydrogen peroxide which may lead to intergranular stress corrosion cracking (IGSCC) of the stainless steel.
IGSCC can be mitigated by lowering the concentrations of ionic impurities and oxidizing species in the reactor water. This may be effected using hydrogen water chemistry (HWC) in which hydrogen is added to the feed water of the reactor. The primary purpose of the added hydrogen is to reduce the dissolved oxidant concentrations and thereby lower the ECP below a critical value at which IGSCC susceptibility is significantly reduced.
Various forms of ECP sensors are used for measuring ECPs in the reactor. The sensors have different configurations for measuring ECPs, and are subject to different problems which affect their useful lives. The useful life should cover the duration of at least a single fuel cycle which is about eighteen months in the United States. However, experience in actual nuclear reactors has demonstrated sensor failure in a shorter duration due to various causes.
U.S. Pat. Nos. 5,848,113 and 5,896,432, commonly owned by the present assignee, disclose and claim two different types of ECP sensors specifically configured for solving corresponding failure problems during operation.
A different type of ECP sensor includes a ceramic probe in which is packed a mixture of metal and metal oxide powder for providing a corresponding reference ECP. This mixture may include iron and iron oxide (Fe/Fe3O4), or copper and copper oxide (Cu/Cu2O), or nickel and nickel oxide (Ni/NiO).
In this type of sensor, the probe is typically in the form of a zirconia tube brazed to a support tube made of a suitable metal such as Invar or xe2x80x9calloy 42xe2x80x9d, which in turn is welded to a stainless steel tube. An electrical conductor extends through the tubes into the probe and is buried in the operative mixture.
In one example, the ceramic probe is formed of magnesia-stabilized-zirconia (MSZ) brazed to an alloy 42 support tube. Since the ceramic probe and metal tube have different coefficients of thermal expansion, they are subject to thermal shock during high temperature operation inside the nuclear reactor which can lead to cracking of the braze joint. The braze material is also subject to corrosion during operation. Both problems limit the useful life of the sensor, since failure of the braze joint causes water leakage inside the sensor and failure thereof.
Accordingly, it is desired to provide an improved ECP sensor addressing these problems.
An ECP sensor includes a tubular ceramic probe having a closed tip at one end packed with a metal and metal oxide powder. A metal support tube receives an opposite end of the probe, and is joined thereto by a braze joint therewith. An electrical conductor extends through the support tube and probe, and has an end buried in the powder for electrical contact therewith. A ceramic band bridges the probe and tube at the joint for sealing thereof.